1. Field of the Invention
The present invention relates to a differential-pressure flow meter and a flow-rate controller for use in a fluid transport pipe in various industrial fields including chemical plants, semiconductor manufacturing, foods, and biotechnology.
This application is based on Japanese Patent Application No. 2008-178050, the content of which is incorporated herein by reference.
2. Description of Related Art
There is a differential-pressure flow meter conventionally and widely used for measuring a flow rate of a fluid. Such a differential-pressure flow meter includes pressure sensors in front of as well as in the rear of an orifice, and is used at a position to measure a pressure of a fluid containing a corrosive chemical such as nitric acid, hydrochloric acid, and hydrofluoric acid, of high purity, as in a semiconductor manufacturing process. Specifically, there is used a chemical solution containing hydrofluoric acid or the like upon etching a semiconductor substrate in a semiconductor manufacturing apparatus. In order to stably supply this chemical solution, such a differential-pressure flow meter is incorporated in a circulation loop of the chemical solution.
This differential-pressure flow meter includes the pair of pressure sensors respectively located upstream and downstream of the orifice in a main fluid channel, and is thus used for calculation of a flow rate by converting a difference in pressure between the two pressure sensors. The differential-pressure flow meter functions as a flow-rate controller that performs feedback control of the flow rate in the main fluid channel to have a desired value by controlling an opening of a variable valve so as to change the flow rate calculated by the flow meter to be equal to a preliminarily set flow rate (refer to Japanese Unexamined Patent Application, Publication No. Hei 5-233068, FIG. 1, or the like).
In the differential-pressure flow meter and the flow-rate controller described above, what is important as a preliminary condition for accurate flow rate measurement as well as accurate flow rate control is to maintain a constant opening of the orifice necessary for measurement of the difference in pressure.
In a case of using a slurry fluid, which can be easily solidified, the diameter of the orifice is sometimes reduced due to a solid substance that is generated by solidification of the fluid and adheres to the orifice. Such adhesion of the solid substance causes an erroneous difference in measured flow rate. In order to avoid such an erroneous difference, when there is an extraneous material such as a solid substance adhering to an orifice, the solid substance is usually washed away in rinse with use of a fluid cleaner flowing in a same direction as the chemical solution flows.
However, the solid substance generated from the chemical solution or the like adheres so strongly that it often remains unremoved by such an ordinary cleaning treatment. Accordingly, when the solid substance from the chemical solution adheres to the orifice, required is a cleaning treatment of disassembling and washing the peripheral part of the orifice in the differential-pressure flow meter or a reverse-current cleaning, in which a fluid cleaner flows in a direction opposite to the normal flow direction. It is pointed out that such a disassembling treatment and a reverse cleaning treatment require much time and effort. Furthermore, the reverse cleaning treatment of flowing a fluid cleaner in the direction opposite to the normal direction is impractical since the fluid cleaner flows in the direction opposite to the normally expected flow directon also in adjacent devices, resulting in unexpected troubles.